to elaborate on what tony said, nirvanix is a secondary storage option whereas the screenshot from the wizard you're refering to is for the primary storage location. after you configure the primary storage device (for example NTFS) you can then go into the properties of the partition and configure nirvanix as the secondary storage device.
from the technote:
Configuring Nirvanix SDN as secondary storage
Use the Enterprise Vault Administration Console to enable and configure the
Nirvanix SDN as secondary storage.
For each vault store partition for which you want to configure Nirvanix as
secondary storage, follow these steps.
To configure Nirvanix SDN as secondary storage
1 Start the Enterprise Vault Administration Console.
2 In the left pane of the Administration Console, expand the Enterprise Vault
site hierarchy until the vault store partition from which you want to migrate
data is visible.
3 Right-click the vault store partition and click Properties. You can also
double-click the vault store partition to open the vault store partition
properties page.
4 In the Collection tab, do the following:
■ Check Use collection files to enable collection.
3 Chapter
■ Select Enterprise Vault as the file collection software.
■ Specify the daily collection times and the amount of time that must elapse
since items were archived before they are eligible for collection.
Note: For optimal performance, do not change the default collection file size.
You can also enable and configure collection when creating a new vault store
partition using the New Partition wizard.
5 In the Migration tab, do the following:
■ Check Migrate files to enable migration.
■ Select Nirvanix Storage Delivery Network as the migrator software.
■ Specify the amount of time that must elapse since a collection file was
created before it is eligible for migration.
■ Choose whether to remove the collection files from primary storage after
they have been migrated. Specify the amount of time that must elapse
since the migration before the files are deleted.
You can also enable and configure migration when creating a new vault store
partition using the New Partition wizard.
6 In the Advanced tab, configure the Nirvanix storage server